1. Field of the Invention
The present invention relates to a voice coil linear actuator and in particular to a small, high-performance linear actuator used in a hydraulic/pneumatic control apparatus or in a precise positioning device such as a hard disk drive.
2. Related Art
One example of a conventional linear actuator will be described with reference to FIG. 13. As a fixed part, a yoke part 55 is formed by disposing a pair of ring-shaped permanent magnets 51 with the same poles facing one another, disposing a pole shoe 52 on one side of each magnet 51 and a side yoke 53 on the other sides, and disposing a tubular yoke 54 between the respective side yokes 53. As a moving part, a coil support 56 is provided in a hollow 61 provided between the facing pole shoes 52 of the yoke part 55. On the outer circumference of the coil support 56, a coil bobbin 57 is disposed in a gap 62 between the circumferential surfaces of the pole shoes 52 and an inner circumferential surface of the tubular yoke 54, with a moving coil 58 being wound around the coil bobbin 57. A cylindrical output shaft 59 is integrally fitted onto inner circumferential surfaces of the coil supports 56. The output shaft 59 passes through the yoke part 55 and is supported via bearings 60 provided in the through-hole between the side yokes 53 on both sides so as to be slidable. A lead wire 63 for supplying electricity to the moving coil 58 is disposed via an internal space in the output shaft 59 and the hollow 61.
When a current flows through the moving coil 58, thrust is produced in the coil support 56 and the output shaft 59 in the axial direction due to an electromagnetic force received in a direction perpendicular to the magnetic fields formed by the magnetic circuits in the yoke part 55 (i.e., the magnetic fields formed between the outer circumferential surfaces of the pole shoes 52 and the inner circumferential surface of the tubular yoke 54), and by switching the direction in which the current flows through the moving coil 58, the output shaft 59 is moved reciprocally in the axial direction (see Japanese Laid-Open Patent Publication No. H06-133523).
In the linear actuator shown in FIG. 13 described above, the coil support 56 is disposed in the hollow 61 provided between the ring-shaped pole shoes 52, and since the range of movement of the output shaft 59 is restricted by this hollow 61, it is not possible to reduce the gap between the two pole shoes 52, thereby placing a limit on miniaturization. Also, in the magnetic circuits formed in the yoke part 55 in rings from the permanent magnets 51, in addition to the magnetic flux produced between the pole shoes 52 and the tubular yoke 54, magnetic flux that leaks from end surfaces aside from the outer circumferential surfaces of the pole shoes 52 is produced. Although the amount of magnetic flux that interlinks the moving coil 58 and the leak magnetic flux produced from end surfaces aside from the outer circumferential surfaces of the pole shoes 52 is low, the direction of interlinking with the moving coil 58 is not necessarily perpendicular and electromagnetic force components in directions aside from the axial direction act upon the moving coil 58, so that the thrust of the moving part (the coil support 56 and the output shaft 59) is reduced and vibration is produced in the moving part in the radial direction.
The output shaft 59 is supported by the bearings 60 provided in through-holes of the side yokes 53, and the yoke part 55 is formed by attaching the tubular yoke 54 so as to fit within the outer diameter of the side yokes 53, with the pole shoes 52 and the side yokes 53 being coaxially attached to the ring-shaped permanent magnets 51. In this way, since the positions used for alignment when assembling the respective components that form the yoke part 55 differ, it is difficult to assemble the parts so that favorable coaxial alignment with the output shaft 59 is maintained for the outer circumferential surfaces of the pole shoes 52 and the inner circumferential surface of the tubular yoke 54. This means that it is difficult to accurately maintain the position and posture of the coil bobbin 57 and the moving coil 58 provided in the gap between the outer circumferential surfaces of the pole shoes 52 and the inner circumferential surface of the tubular yoke 54. Therefore, to prevent the yoke part 55 from interfering with the coil bobbin 57 and the moving coil 58, it is necessary to increase the clearance between the coil bobbin 57 and the pole shoes 52 and tubular yoke 54. However, if the gap between the pole shoes 52 and the tubular yoke 54 is increased, the leak magnetic flux from the magnetic circuits increases, so that a large thrust cannot be produced in the moving part in the axial direction.
To achieve a sufficient movement range for the coil support 56, it is necessary to attach the pole shoes 52 and the side yokes 53 to the permanent magnets 51 so that the ring-shaped pole shoes 52 disposed on both sides have a gap in between. In view of decreases in magnetic characteristics of the magnets and corrosion resistance, bonding using adhesive is normally used as the means of attachment between magnet 51 and yokes (pole shoes 52 and side yoke 53). However, to reliably harden the adhesive and maintain a sufficient bonding strength, it becomes necessary to carry out a heat treatment on the bonded parts, so that there is the problem that the number of assembly processes and assembly time increase, resulting in low productivity.
Also, as shown in FIG. 14, the output shaft 59 of the moving part is inserted through a guide pipe 64 fixed to a fixed yoke (not shown), so that when the output shaft 59 is guided by the guide pipe 64 and moves reciprocally in the axial direction, the output shaft 59 slides on the inner wall surface of the guide pipe 64. At this time, there is the risk of the output shaft 59 becoming inclined and sliding in point contact near openings 65 at both ends of the guide pipe 64. If this happens, the load is concentrated at the contacting parts of the guide pipe 64 and the output shaft 59, which accelerates abrasion.